In recent years, the requirements on the quality of ferrous metal materials such as the precision of plate thickness, material quality, surface grade, etc. have become increasingly severe. In particular, in the case of production of thick-gauge steel plate, a cooling control process using on-line control aimed at reduction of the alloy ingredients, streamlining of the heat treatment process, etc. has been employed. This cooling control process using on-line control cools the thick-gauge steel plate, which is in a high temperature state after hot rolling, immediately or ten or so seconds later by a predetermined cooling rate until a predetermined cooling stop temperature so as to reduce the content of expensive alloy ingredients and still obtain the desired strength, toughness, and rigidity. Usually, in this process, the amount of cooling water and the water cooling time are adjusted to control the cooling rate and the cooling stop temperature respectively.
As the apparatuses used for the above controlled cooling, in general the slit jet cooling method and multi mist jet cooling method are employed, but there are the problems of a narrow effective cooling area, maintenance of a slit clearance in the plate width direction of several millimeters at all times, the location of use, etc. Aerated water cooling nozzles with broad ranges of control of the amounts of water are being extensively employed, but for stable and uniform cooling of steel plate, a high density nozzle arrangement is desirable, but this nozzle configuration and the increased complexity make a box type cooling apparatus necessary. This box type cooling apparatus has the advantage of use of nozzles of a type directly feeding water or air to a plurality of nozzles from a water or air header and therefore a simple structure. As the nozzle configuration of this box type cooling apparatus, as disclosed in Japanese Utility Model Publication (A) No. 63-111208, there is proposed a cooling apparatus comprised of outside and inside headers having respectively independent cooling water feed systems wherein the outside header is provided with a large number of bell mouth nozzles having constricted parts of predetermined angles at the cooling water spray surface and the inside header is provided with a large number of nozzles having tapered curved front ends so as to generate ring shaped impact flows and, as disclosed in Japanese Utility Model Publication (A) No. 1-135108, there is proposed a method of obtaining an excellent cooling performance even under conditions where the pressure of a directly impacting flow and the pressure of a tapered slanted flow are the same by making the orifice size at a discharge port of a bell mouth nozzle simultaneously spraying a directly impacting flow and a slanted flow smaller than the orifice size of a direct impact flow introduction port and tapering the introduction port of the slanted flow.